This invention relates to a thermally actuated switch, and more particularly to a one-time or nonresettable thermally actuated switch.
Generally, such one-time switches are used to protect a motor, transformer, or other electrical apparatus from operating in ambient temperatures which exceed a predetermined maximum safe level. Such one-shot switches are typically used as back-up devices for protecting electrical apparatus which may also have current sensing devices or other thermostatic switches or the like adapted to provide normal overload protection for the apparatus, the switch of this invention functioning where the normally relied upon devices might fail. For example, such a switch may be serially connected to the windings of a motor so as to protect the motor by interrupting the power supplied thereto in the event that, because of overheating of the motor winding or for any other reason, the ambient temperature to which the switch is exposed exceeds a level which could cause damage to the motor. These switches are often relatively small (e.g., 0.50 inches or 1.27 cm. long and 0.135 inches or 0.34 cm. in diameter) so they may be readily incorporated in the apparatus they are to protect so as to be responsive to the ambient temperatures to which the apparatus is exposed and to be responsive to any abnormal rise in temperature the apparatus might undergo due to overload or short circuit conditions in the apparatus.
More particularly, such switches utilize a fusible material which rapidly undergoes a change of state when heated to a predetermined temperature (i.e., the fusible material melts) to effect breaking of the circuit. Reference may be made to such U.S. Pat. Nos. as 3,180,958, 3,291,945, 3,309,481, 3,505,630 and 3,519,972 for such one time thermally actuated switches.